With the development of multimedia technology, digital still cameras, digital video cameras and mobile phones with camera functions have gained increasing popularity among consumers. In addition to their pursuit for the miniaturization of these devices, customers are also demanding for an increasing improvement in the quality (i.e., clarity) of images captured by such devices. On the other hand, whether such a device could capture a high imaging quality heavily depends on what components the device incorporates. In particular, the imaging quality of a camera is determined by an incorporated imaging sensor, which is a critical component of the camera.
Nowadays, most image sensors are implemented as complementary metal-oxide-semiconductor (CMOS) circuits. In a typical CMOS sensor, each pixel generally includes a photodiode and other circuitry components, arranged in a sensor layer overlying a substrate. The sensor layer is usually covered by one or more dielectric layers selectively laminated with several metal interconnect layers which incorporate additional circuitry components and are intended to form interconnect structures. The CMOS sensor further includes a pad for electrically interconnecting the circuitry components. In general terms, the side of the CMOS sensor where the dielectric layers and associated interconnect structures are formed is referred to as a front side, while the opposite side of the substrate is referred as a backside.
For a front side-illuminated (FSI) CMOS sensor, light from an illuminated scene is incident on its front side when the sensor is used, while backside-illuminated (BSI) CMOS sensors are those with their backsides receiving the light from the scene when in use. Any CMOS sensor, no matter it is an FSI or BSI one, should be equipped with optical filters to deal with the light from the illuminated scene before the light enters corresponding photodiodes.
In a conventional CMOS sensor, in order to prevent the occurrence of optical cross-talk between adjacent optical filters, light-blocking layers are further arranged therebetween. However, as the light-blocking layers and the pad are conventionally formed in different processes using different techniques, the sensor fabrication process is associated with high complexity and cost and suffers from a low throughput. Another deficiency of the conventional CMOS sensor is that a protection layer for the pad, which causes reduction of light absorption, intervenes between the optical filters and the photodiodes.